It is recognized that drilling dissimilar materials calls for speeds and feeds appropriate for each of the particular material. For instance, it is known that all materials when drilled with a given feed rate, such as inches per revolution, generate certain amount of thrust, which is resistance to drill penetration. The thrust, or resistance, is normally higher as the hardness of the material increases, and hard materials also require a slower speed or rotation (RPM). The correlation of that is as the material gets softer the thrust decreases and the rotation can increase. When drilling titanium the thrust is quite high and the drill rotation is low, but when drilling aluminum the thrust is low and the drill rotation is high. There are a number of non-metallic materials which are frequently used in the production of products, whether in the aircraft field or in other fields. These materials are graphite or epoxy which are in the class of soft materials which can be drilled at higher speeds than steel or titanium and materials of that class of hardness.
When drilling one type of material, the drill thrust or feed and speed can be preset at the start and maintained substantially unchanged throughout the depth of drill. However, it frequently happens that dissimilar materials need to be assembled and drilled for the insertion of securing means to hold the material. The drill therefore needs to be capable of changes in speeds, and frequently the speed is set for the material that calls for the lowest speed. This is inefficient and hard on the drill bit.
An early appreciation of the speed and feed problem encountered in drilling materials varying in hardness is found in Gerentes U.S. Pat. No. 2,547,079 issued Apr. 3, 1951 which related to an electric drill used in mines and quarries. In this machine, the object is to depend upon two independent motor driven mechanism, one for the feed movement and the other for the rotation of the drill bit. Somewhat the same idea may be found in Adams et al U.S. Pat. No. 3,526,158 issued Sept. 1, 1970 wherein two motors are incorporated to drive a drill and countersink, and a mechanical feeler is used to sense the drill break-through penetration of a workpiece and to shift to the slower speed motor to drive the countersink. Other prior examples are described by Reynolds in U.S. Pat. No. 3,224,338 issued Dec. 21, 1965; in U.S. Pat. No. 3,325,710 issued June 13, 1967; and in U.S. Pat. No. 3,248,629 issued Apr. 26, 1966; each of which is concerned with speed control of an electric drive motor for a cutting tool so that the work factor remains substantially constant while speed and torque vary inversely. However, these prior art examples of the work of Reynolds are not concerned with the sequential drilling of dissimilar materials in one drill pass.
The conventional machine practice places the speed and feed selection in the judgment of the operator as to what he thinks the material requires. While the concepts disclosed by Gerentes or Adams et al might be thought of as replacing the experience of the operator, neither has proved practical in the light of current needs. However, the work of Emerson et al as disclosed in U.S. Pat. No. 3,418,549 which issued Dec. 24, 1968 took into account the speed and feed variations when sequentially drilling materials of different hardness characteristics. This feature took the form of control of the cutter machine slide feed rate by utilizing a voltage proportional to the spindle motor torque to adjust a manual speed rate override in accordance with variations in the load sensed by the cutter. This is attained by establishing certain predetermined spindle motor speed-torque droop characteristics sufficient to meet the torque required by the cutter for each particular material. In short, this patent is directed to a machine operation combining numerical and torque control systems in which a punched tape is used in conjunction with manual feed rate control which supplies a variable voltage to a clock to vary the frequency of its variable oscillator. The system requires an interpolator which translates data from the clock and from the tape into velocity and position directions for each of the X, Y and Z axes of the machine.